Does this line of light turn as you turn the eyepiece in the diagonal while looking thru it ? if it does its the eyepiece , if not its the scope or diagonal ( have a good look at the mirrored surface , but if you see something DONT TOUCH IT ! or you will void warranty ) .
Can you take a photo of what you are describing and post it here ?, just hold a phone camera up to the eyepiece and snap away , one will turn out ok .
Brian.